Here is my standard short answer to the young-Earth creationist claim that the deposits formed by the 1980s eruptions of Mt St Helens demonstrate that Noah’s flood could be responsible for the sedimentary rock record.

The deposits of Mt St Helens (MSH) demonstrate that volcanoes can do a lot of geologic work in a short amount of time. This comes as a surprise to no one. Any good volcanologist or sedimentologist will be able to recognize volcaniclastic rocks in the rock record. In fact these types of deposits are quite common in the rock record, and are thousands of meters thick in places. For instance, there are the deeply-eroded remains of a large Cretaceous stratovolcano (named the Sliderock Volcano) not too far from where I live in south-central Montana. This is recognized as a stratovolcano by having the remains of a magma chamber in the center, and then volcaniclastic rocks dipping away from that central vent area in all directions. The mountain was probably the size of the larger Cascade Range volcanoes, such as Shasta or Rainier. The YEC claim that this entire complex volcano formed and eroded in a few weeks towards the end of the flood is mind-boggling.

Most of the sedimentary rocks of the geologic record are actually quite unlike the volcaniclastic rocks of MSH.

Most sandstones and conglomerates are nothing like the deposits of MSH.

No shales are like the deposits of MSH.

No limestones are like the deposits of MSH.

No evaporites are like the deposits of MSH.

Conclusion: most of the rock record was formed in settings that were not at all like MSH.

As a Bible-believing Christian, I recognize that the flood account in Genesis says nothing about stratovolcanoes in Washington (MSH, part of the research topic for my M.S. in geology) or Montana (the Sliderock Volcano I referred to), so I do not have to try to squeeze such events into Scripture. The truthfulness of the Bible does not depend on YEC flood geology being correct.